1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly to a method of manufacturing a semiconductor device, including crystallizing at least part of a non-monocrystalline semiconductor thin film.
Generally, even a single crystal involves disturbance of atomic rows (such as dislocation), and it is difficult to distinguish “single crystal” from “crystal close to single crystal,” and thus, it should be noted in this specification that “crystal close to single crystal” is also described as “single crystal.”
2. Description of the Related Art
The SOI (silicon on insulator) technology for forming a monocrystalline silicon on an insulating material substrate or on an insulating film is known as technology for realizing ULSI (ultra large-scale integrated circuit) integration, low power consumption, and high speed. This technology is classified into (1) a method of forming a single crystal thin film on an insulating film formed on a single crystal semiconductor wafer, for example, a silicon wafer, and (2) a method of crystallizing or re-crystallizing a non-monocrystalline (amorphous or polycrystalline) semiconductor thin film, for example, a non-monocrystalline silicon thin film, formed on an insulating material substrate or an insulating film. In both methods, it is very important to enhance the degree of crystallinity of silicon. Preferably, a region for forming a transistor should be single crystal, the crystal plane orientation should be uniform, in particular, the surface should be (001) plane, and the crystal orientation in the current flowing direction should be (100) plane. Accordingly, the method (1) is widely employed, separation by implanted oxygen (SIMOX) using a monocrystalline silicon wafer or a wafer bonding.
On the other hand, the method (2) is not employed in today's silicon ULSI technology. However, since the substrate material used is not limited, the method (2) can be applied in various electronic elements or electronic devices if a single crystal semiconductor thin film, such as a single crystal silicon thin film, of high quality can be formed. Accordingly, it is earnestly demanded to improve the method (2).
In the 1980s, many studies have been conducted in an attempt to form monocrystalline silicon thin film having uniform plane orientation. Of these, zone melting technology by radio frequency induction heating is an important technology, and is known as a technology capable of forming a monocrystalline silicon rectangular region of which crystal orientation has (001) plane.
As reported by Akira Fukami and Yu Kobayashi in “Journal of Electronic Communications Society” (1986/9 vol. J69-C No. 9, pp. 1089-1095), in the zone melting method, first, a polycrystalline Si thin film is deposited on a quartz substrate by an atmospheric chemical vapor deposition (CVD) method, and the thin film is patterned to obtain a pattern in which a number of rectangular regions spaced from each other and arranged linearly are mutually linked by thin necks. Then, an elongated radio frequency induction heater is positioned at the backside of the quartz substrate to heat the linearly arranged rectangular regions sequentially to 1412° C. or more to melt the polycrystalline silicon placed in a position corresponding to the heater, forming molten silicon region. Next, the heater is moved in the array direction of the rectangular regions, whereby the polycrystalline silicons are sequentially melted, and the entire rectangular region is melted. Of the rectangular region, the portion heated by the heater and then cooled has been already monocrystallized, and the other portion is not crystallized. By changing the dimensions (length and width) of the necks, heat flow is changed locally, and the crystal orientation varies depending on the heat flow. By optimizing the length and width of the necks, a crystallized rectangular region having orientation of (001) plane can be formed.
Incidentally, the technology for forming a crystallized silicon thin film on a glass or plastic substrate is applied in a technology for enhancing the performance of a thin film transistor used in a driving element of a liquid crystal display or the like. For example, when a semiconductor layer of the thin film transistor is changed from an amorphous structure to a polycrystalline structure, the mobility of the transistor becomes 100 times or higher.
In this case, however, at the time of crystallization, due attention must be paid to thermal damage on the substrate (for example, heating temperature for crystallization must be 600° C. or less in a general glass substrate, or 150° C. or less in plastics).
In the zone melting method using the radio frequency induction heating, the substrate (quartz substrate) is partly heated to temperature exceeding a melting point of silicon (1410° C.), and therefore, it cannot be applied in the field of liquid crystal display, in which the substrate is formed of a low melting point material such as glass or plastics.
To align the crystallized film in (001) plane orientation, it is required to optimize the shape of the necks connecting the rectangular silicon regions, which limits layout of transistors and circuits to be formed later.
Accordingly, as a method of crystallizing an amorphous silicon thin film without thermally damaging a substrate, an excimer laser crystallization method has been developed. In this technology, excimer laser beam is adjusted by a homogenizing optical system so that the intensity is uniform on the section, and the beam is shaped into a rectangular form (for example, a sectional shape of 150 mm×200 μm) through a metal mask having an opening of elongated rectangular shape. With this shaped laser beam, a surface of an amorphous silicon thin film deposited on a glass substrate is scanned at right angle to the longer side direction of the rectangle, and irradiated with laser in the shorter side direction at intervals of 10 μm. The silicon thin film having absorbed the laser beam is melted, and cooled to be polycrystalline silicon. In this technology, the substrate is not damaged thermally even if a general glass or plastic substrate is used. This is because the excimer laser is a pulse laser having a pulse width of about 20 ns, and the crystallization is complete in about 50 to 100 ns. The obtained crystal grain size depends on the laser energy density, and a polycrystalline thin film formed of crystal grains having grain size of about 0.1 to 1 μm can be formed. As for the plane orientation, it is reported that crystal grains formed by single laser irradiation are not aligned, but that, when laser irradiation is repeated hundreds of times, the surface orientation is aligned to (001) plane or (111) plane (as for the former, see, for example, D. P. Gosain, A. Machida, T. Fujino, Y. Hitsuda, K. Nakano and J. Sato, “Formation of (100)-Textured Si Film Using an Excimer Laser on a Glass Substrate,” Jpn. J. Appl. Phys., Vol. 42 (2003), pp. L135-L.137; as for the latter, see, for example, H. Kuriyama, et al., “Enlargement of Poly-Si Film Grain Size by Excimer Laser Annealing and Its Application to High-Performance Poly-Si Thin Film Transistor,” Jpn. J. Appl. Phys., Vol. 30 (1991), pp. 3700-3703).
However, in the excimer laser crystallization method, the crystallinity in the individual crystal grains may be made monocrystalline, but the thin film as a whole is polycrystalline. Therefore, when multiple transistors are formed, grain boundaries are present in the channel regions, so that the mobility is lowered, and the performances (threshold voltage, sub-shred coefficient, mobility) fluctuate among transistors. To increase the crystal grain size, the laser fluence (energy density) must be set at a level as closer as possible to the critical fluence at which the silicon thin film is totally melted. However, when the laser fluence exceeds the total melting condition, the silicon thin film becomes very fine crystals, which is not preferred. In other words, tolerance of the laser fluence to fluctuations is narrow. Since the crystal grain size is about 1 to 2 μm at maximum, there occurs limitation that the transistor size must be controlled smaller. For example, when a large area substrate for display of about 1 m×1 m is used, an extremely advanced fine processing technology is required. Besides, to align the surface orientation to (001), the laser must be irradiated 200 times or more (or for (111) plane, about 10 times). Hence, a very long processing time is required for crystallization. Even if the orientation of the surfaces of individual crystal grains, which provide the upper side (one face) of the crystallized film, is aligned uniformly at (001), the configuration is a disorderly rotation about the surface axis, and the crystal orientation of the section of the thin film is not aligned. That is, the plane orthogonal to the surface of the crystallized film cannot be oriented to (001) orientation.
Further, crystallization using a flash lamp alone instead of the excimer laser has been attempted. However, when multiple transistors are formed, however, grain boundaries are formed in the channel regions, so that the mobility is lowed, and the performances (threshold voltage, sub-shred coefficient, mobility) fluctuate among transistors, though the crystallinity within the crystal grains can be made monocrystalline as in the case of the excimer laser crystallization method.
As a technology developed through the excimer laser crystallization technique, a technology called sequential lateral solidification (SLS) is also known. This technology is disclosed in, for example, Japanese Patent No. 3204986. In this technology, as shown in FIG. 12A, an excimer laser beam 11 homogenized in light intensity by the homogenizing optical system is passed through a metal mask 12 having a thin gap of about 2 μm to be shaped into a rectangular shape in section. When the fluence (energy density) of the laser having passed through the gap is set such that an amorphous silicon thin film 13 becomes a molten silicon 14 totally melted in the thickness direction, a lateral growth occurs from the outside region of the gap toward the inside, and a crystallized silicon 16 is formed (FIG. 12B). Next, the target structure is moved to the left direction by 2 μm as indicated by an arrow 17, and laser is emitted. Then, the molten silicon 14 grows in the lateral direction, starting from the seed crystal at the right end of the crystallized silicon 16 formed by the previous laser irradiation (FIG. 12C). By repeating this process of laser irradiation and target moving, a polycrystalline silicon thin film of large grain size can be formed. In this case, the plane shape of the mask 12 is made in a checkered pattern mask 19 as in FIG. 12D. In this case, when laser irradiation is repeated, the processing time is improved, and the overlaying region of crystallization is enhanced, so that a uniform laterally grown polycrystalline thin film is formed on the substrate surface.
However, since, in the SLS method, nearly half of the laser beam is shielded by the metal mask, the laser energy cannot be utilized effectively. As a result, it takes a longer time in processing for crystallization. Besides, since the positions of the crystal grains are scattered, the performance fluctuates among transistors as in the case of the excimer laser crystallization. Hence, the plane orientation of the crystal grains is not uniform, which also leads to fluctuation of performance among transistors.
As a technology further developed through the excimer laser crystallization method, a phase-modulated excimer laser crystallization method is also known (see, for example, Masakiyo Matsumura, “Surface Science,” Vol. 21, No. 5, pp. 278-287, 2000). This method is featured in that, as shown in FIG. 13A, excimer laser beam 21 is passed through an optical component called a phase shifter 22 (for example, a quartz plate processed by forming steps), and therefore, the laser beam intensity distribution is modulated in space as indicated by reference numeral 23 in FIG. 13B. An amorphous silicon thin film 24 is irradiated once by using the thus modulated laser beam, and the irradiated region 25 is crystallized as shown in FIG. 13C.
This method, different from the excimer laser crystallization method or SLS method, does not use a uniform light intensity distribution, and does not require multiple times of laser irradiation. In this method, the modulated light intensity distribution 23 provides an inclined temperature distribution in the thin film irradiated with the laser, and a crystal nucleus is formed at the position 27 of small energy, so that the position of crystal nucleus can be accurately determined. Moreover, as shown in FIG. 13D, crystal grains 26a, 26a of large grain size can be obtained by lateral growth from the crystal nucleus. By this method, crystal grains of large grain size are formed, and the position of crystal nucleus can be also controlled.
However, in the phase modulated excimer laser crystallization technology, crystal grains of large grain size are obtained, but it has been further demanded to form larger crystal grains so as to fabricate a plurality of transistors in one crystal grain, and to relax the restriction on the circuit layout.